1. Field of the Invention
The present invention relates to an apparatus for classifying solids and more particularly to a cylindrical classifier for classifying chipped wood by size.
2. Description of the Related Art
Classifying machines are frequently used to separate specific sizes of solids from other sizes in processing systems. For example, classifiers are commonly used to classify crushed stone in stone crushing processes or to classify wood chips in wood chip processing systems.
A common wood chipping system includes a wood chipper, for example, of the type exemplified by U.S. Pat. No. 3,069,101 to Wexell. The wood chipper chips large logs, timber, branches and other wood scraps into smaller chips. The chips are exhausted by the chipper onto a conveyor or into a receptacle. The chips thus formed are typically used for making paper. Paper production requires that the wood chips be converted into pulp by chemical digestion. The rate of chemical digestion of a wood chip is directly related to the thickness of the chip. The width and length of the chips is relatively unimportant because the diffusion of chemicals into the wood is most important in the direction of the thickness of the chip. Thus, there is an ideal range of thickness for chips for paper making. However, chips produced by wood chippers are not all uniform in size and shape. Indeed, due to the nature of the chipping process and the diverse physical characteristics of the wood, the chips produced may be quite diverse in size, shape and thickness. The output of a conventional chipper includes oversized chips and also undersized particles, such as fines and pins. The oversize chips are not properly digested by the chemicals because of their great size. The undersize pins and fines also are disadvantageous to the process.
Therefore, to maintain proper quality control and optimum consistency for further processing, the chips exhausted from the chipper must be classified by size to separate the acceptably sized chips (accepts) from the oversized material (overs) and undersized material (fines).
Conventional classifiers use screens of various aperture size. In some such classifiers, the screens are stacked with the largest on the top and the most fine on the bottom. Such a classifier will separate solids on the basis of size, leaving the overs on the top screen while allowing fines to fall to successively smaller screens, leaving only the smallest parts to fall through the lowest and finest screen. A vibration mechanism is typically provided to encourage the movement of the material being classified through the screens.
Other classifiers are in the form of cylindrical screens which are inclined and rotate to encourage movement of the material and complete classification during residency in the classifier. Known cylindrical classifiers are provided for the separation of fines, with overs being previously or subsequently separated from the accepts.
Screen-type classifiers classify solids on the basis of overall size. Thus, such classifiers will not be able to classify objects on the basis of one dimension alone. For example, screen type classifier will not be able to classify wood chips on the basis of thickness, regardless of the width or length.